For guts adapted for being incorporated in rackets used for sports, there has been used animal strings such as sheep guts, whale string or the like which is properly twisted and is then subjected to surface treatment.
This animal string gut is good in tensile modulus but is not resistant to moisture and becomes sticky at its surface and easily stretches under wet conditions and, on the other hand, contracts when it becomes dry. If the gut is stretched in a racket frame under extreme wet conditions, it is stretched into its most extended state. Thus, it often happens that the same is broken when dried.
Additionally, the available amount thereof is limited from a standpoint of resources, and difficulty with respect to supply sources is involved. Further, such gut is deficient in that it is comparatively small in breaking strength and weak in durability, and in that it is difficult to make products constant in modulus property on account of the varying character of the raw material. Moreover, the gut varies in its size and shape so that, when a tensile load is applied thereto, the force is likely to concentrate at its smaller diameter portions and breakage occurs at that portion. Additionally, the gut is high in price. (It will be noted relative to the above that the modulus is a value expressed as W/.DELTA.1/1 wherein the tension load is W and an elongation of the gut at that time is .DELTA.1/1 .times. 100 (%).)
Recently, a gut for a racket made of synthetic fiber has been developed. This kind of gut can overcome various deficiencies of animal string gut with respect to moisture resisting properties, inequalities, availability, price and so forth. However, it is lower in tensile modulus and is defective with respect to changing of characteristic of elongation in response to tensile load, as compared with animal string, so that proper adjustment thereof on applying the same to a racket frame cannot be easily obtained. Moreover, it is inferior to animal string gut on the basis of ball batting properties and bouncing properties.
More specifically, the tensile modulus of the different animal string (that is, of sheep gut and of whale string) extend nearly linearly in almost direct proportion to the tensile load as will be shown hereinafter. However, in the case of synthetic fiber -- (for instance, Nylon fiber), it fluctuates and is not linear. Also, it is lower in tensile modulus than the typical sheep gut, and thus is not suitable for use in a racket.
There has been also proposed a gut for tennis such that the surface of a synthetic resin filament is made rough and there is applied thereto a treating agent comprising paste having an adhesion property and a small amount of filler serving to give a frictional characteristic. Thereby, the ball batting property is improved to have a sufficient ball holding ability. There has also been used a gut for tennis and badminton such that a synthetic fiber multifilament is twisted and the surfaces of the monofilaments thereof are fused together under stretched condition. None of these guts overcome sufficiently the aforementioned defects inherent in synthetic fibers. They are low in tensile modulus and are inferior to animal string.
For avoiding the defects in synthetic fibers, an improvement has been tried whereby a metallic wire is embodied in a synthetic fiber. The metallic wire is small in specific rigidity (Young's modulus/specific gravity). If the same is combined to such an extent that an appreciable result is obtained, the resultant gut is large in weight and, at the same time, lacks flexibility. Accordingly, for preventing any increase in weight of the associated racket, the gut must be made smaller in diameter or the manner of stretching the gut in a racket frame must be changed.